The primary cause for osteosarcoma, the most common form of primary bone cancer, is not known. Researchers believe that infection by the SV40 virus is a possible contributing cause, since recent investigations have identified SV40 DNA sequences in tissue samples from osteosarcomas, but no SV40 DNA sequences were found in healthy tissue. SV40 is a monkey virus; however, in 1960 some polio vaccine was contaminated with the virus. Researchers hypothesize that this vaccine was the source of the virus found in osteosarcomas decades later.
Which of the following, if true, most strongly supports the researchers' hypothesis?